Experimental autoimmune encephalomyelitis (EAE) is an animal model for human multiple sclerosis (MS). The development of EAE relies on the generation of an autoimmune T cell repertoire and the activation of myelin antigen-specific T cells in the peripheral lymphoid organs. This application is inspired by our finding that CD24 is required for the myelin oligodendrocyte glycoprotein (MOG) induced EAE. We have recently found that CD24 is critically involved in the thymic generation of MOGspecific T cells. Specific aim 1 is to test the hypothesis that CD24 is required for the generation of myelin antigen, but not foreign antigen specific T cells. We have recently bred 2D2 TCR transgenic mice (TCR specific for MOG35-55) with CD24-/- mice and generated CD24-deficient, TCR transgenic mice (2D2/CD24-/-). 2D2/CD24-/- mice have atrophic thymi and mature 2D2 T cells were almost undetectable in their peripheral lymphoid organs. The thymus atrophy/T cell deletion phenotype was not found in CD24-deficient, OT2 and OT1 TCR transgenic mice (TCRs recognize chicken ovalbumin, OVA). We will further determine whether the thymus atrophy/T cell deletion phenotype can be found in CD24-deficient, myelin basic protein (MBP)-specific CD4 and CD8 TCR transgenic mice. Specific aim 2 is to test the hypothesis that CD24 inhibits negative selection of myelin antigen specific immature thymocytes. We will breed 2D2/CD24-/- mice with MOG-/- mice to generate MOG/CD24 double deficient mice to determine whether the thymus atrophy/T cell deletion phenotype in 2D2/CD24-/- mice is MOG antigen dependent. We will inject OVA to CD24-deficient OT2 mice (OT2/CD24-/-) to determine if thymocytes from OT2/CD24-/- mice are more sensitive to deletion/apoptosis than that of OT2/CD24+/+ mice. Additionally, we will compare CD24-/-BALB/c mice with CD24+/+BALB/c mice for the frequencies of viral super antigen specific T cells. Our experiments will reveal a previous unknown function of the role of CD24 in EAE, and probably in MS development.